Method of making an intraluminal stent graft

ABSTRACT

A method of making an tubular intraluminal graft in the form of a tubular diametrically adjustable stent having a tubular covering of porous expanded polytetrafluoroethylene which is less than 0.10 mm thick. The covering may be on the exterior surface of the stent, or on the interior surface of the stent, or both. The covering may be affixed to the stent by an adhesive which is preferably fluorinated ethylene propylene.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/083,461, filed Feb. 25, 2002, (U.S. Pat. No. 6,547,815) which is adivision of U.S. application Ser. No. 09/306,522 filed May 6, 1999 (U.S.Pat. No. 6,357,104) which is a continuation of U.S. application Ser. No.08/872,837 filed Jun. 11, 1997 (U.S. Pat. No. 5,925,075), which is acontinuation of U.S. application Ser. No. 08/109,214 filed Aug. 18, 1993(U.S. Pat. No. 5,735,892).

FIELD OF THE INVENTION

This invention relates to the field of intraluminal grafts andparticularly to a method of making thin-wall intraluminal grafts usefulas an inner lining for blood vessels or other body conduits.

BACKGROUND OF THE INVENTION

Conventional vascular grafts have long been used for vascular repair inhumans and animals. These devices are typically flexible tubes of wovenor knitted polyethylene terephthalate or of porouspolytetrafluoroethylene (hereinafter PTFE). Grafts of biological originare also used, these being typically fixed human umbilical or bovinearteries. These conventional vascular grafts usually require invasivesurgical methods that expose at least both ends of the segment of vesselto be repaired. Frequently it is necessary to expose the entire lengthof the vessel segment. These types of repairs consequently cause majortrauma to the patient with corresponding lengthy recovery periods andmay result in occasional mortality.

Alternative methods have evolved which use intraluminal vascular graftsin the form of adjustable stent structural supports, tubular grafts or acombination of both. These devices are preferably remotely introducedinto a body cavity by the use of a catheter type of delivery system.Alternatively they may be directly implanted by invasive surgery. Theintent of these methods is to maintain patency after an occluded vesselhas been re-opened using balloon angioplasty, laser angioplasty,atherectomy, roto-ablation, invasive surgery, or a combination of thesetreatments.

Intraluminal vascular grafts can also be used to repair aneurysmalvessels, particularly aortic arteries, by inserting an intraluminalvascular graft within the aneurysmal vessel so that the prostheticwithstands the blood pressure forces responsible for creating theaneurysm.

Intraluminal vascular grafts provide a new blood contacting surfacewithin the lumen of a diseased living vessel. Intraluminal grafts arenot, however, limited to blood vessels; other applications includeurinary tracts, biliary ducts, respiratory tracts and the like.

If the intraluminal graft used is of thin enough wall and adequateflexibility, it may be collapsed and inserted into a body conduit at asmaller diameter location remote from the intended repair site. Acatheter type of delivery system is then used to move the intraluminalgraft into the repair site and then expand its diameter appropriately toconform to the inner surface of the living vessel. Various attachmentmethods including the use of adjustable stents may be used to secure theintraluminal graft at the desired location without the necessity ofinvasive surgery.

Intraluminal vascular grafts were suggested as early as 1912 in anarticle by Alexis Carrel (Results of the permanent intubation of thethoracic aorta. Surg., Gyn and Ob. 1912;15:245-248). U.S. Pat. No.3,657,744 to Ersek describes a method of using one or more adjustablestents to secure a flexible fabric vascular graft intraluminally, thegraft and stent having been introduced distally and delivered to thedesired position with a separate delivery system.

Choudhury, U.S. Pat. No. 4,140,126, describes a similar method ofrepairing aortic aneurysms whereby a polyethylene terephthalate vasculargraft is fitted at its ends with metal anchoring pins and pleatedlongitudinally to collapse the graft to a size small enough to allow fordistal introduction.

Rhodes, U.S. Pat. No. 5,122,154 and Lee, U.S. Pat. No. 5,123,917,describe endovascular bypass grafts for intraluminal use which comprisea sleeve having at least two diametrically-expandable stents. Rhodesteaches that the sleeve material is to be made of conventional vasculargraft materials such as GORE-TEX® Vascular Graft (W. L. Gore &Associates, Inc., Flagstaff Ariz.) or Impra® Graft (Impra, Inc. TempeAriz.). Both the GORE-TEX Vascular Graft and Impra Graft are extrudedand longitudinally expanded PTFE tubes. Additionally, the GORE-TEXVascular Graft possesses an exterior helical wrapping of porous expandedPTFE film. The difficulty with the use of either the GORE-TEX VascularGraft or the Impra graft as the sleeve component is that the relativelythick, bulky wall of the extruded, longitudinally expanded PTFE tubeslimits the ability of the tube to be contracted into a smallcross-sectional area for insertion into a blood vessel. For example, thewall thickness of a 6 mm inside diameter Thin Walled GORE-TEX VascularGraft is typically 0.4mm. The thinness of the wall is limited by thedifficulty of manufacturing an extruded, longitudinally expanded tubehaving a thin wall of uniform thickness.

SUMMARY OF THE INVENTION

The present invention is a method of making a tubular intraluminal graftcomprising a tubular, diametrically adjustable stent having an exteriorsurface, a luminal surface and a wall having a multiplicity of openingsthrough the wall, and further having a tubular covering of porousexpanded PTFE film affixed to the stent, said covering being less thanabout 0.10mm thick.

Porous expanded PTFE film has a microstructure of nodes interconnectedby fibrils and is made as taught by U.S. Pat. Nos. 3,953,566; 4,187,390and 4,482,516. As will be described further, the fibrils may beuniaxially oriented, that is, oriented in primarily one direction, ormultiaxially oriented, that is, oriented in more than one direction. Theterm expanded is used herein to refer to porous expanded PTFE. The termsexpand, expanding and expandable are used herein to refer todiametrically adjustable intraluminal stents. More specifically, theterm balloon-adjustable refers to stents of the Palmaz type as taught byU.S. Pat. No. 4,776,337 which typically require a balloon catheter toincrease the diameter of the stent within a blood vessel. The termself-expanding refers to stents which increase in diameter by variousother means. Stents of this type include stents of braided wire made astaught by Wallsten, U.S. Pat. No. 4,544,771; and stents of formed wiremade as taught by Gianturco, U.S. Pat. No. 4,580,568. Stents of thistype expand to a larger diameter after being released from aconstraining force which restricts them to a smaller diameter.Self-expanding stents also include stents formed from Nitinol wire madeas taught by PCT US 92/03481. These stents expand in diameter whenexposed to a slight increase in temperature.

The tubular covering of porous expanded PTFE film may be affixed toeither the exterior surface or the luminal surface of the stent.Alternatively, a first tubular covering of porous expanded PTFE film maybe affixed to the exterior surface of the tubular diametricallyadjustable stent and a second tubular covering of porous expanded PTFEfilm may be affixed to the luminal surface of the tubular diametricallyadjustable stent. The first and second tubular coverings of porousexpanded PTFE film may be affixed to each other through the openingsthrough the wall of the stent.

The porous expanded PTFE film may be affixed to the stent with anadhesive. The adhesive may be a thermoplastic adhesive and morepreferably a thermoplastic fluoropolymer adhesive such as fluorinatedethylene propylene (hereinafter FEP) or perfluoroalkoxy (hereinafterPFA). Where first and second tubular coverings of expanded PTFE film areaffixed to each other through the multiplicity of openings in the stentwall, the two coverings may be affixed by heating them above thecrystalline melt point of the PTFE film adequately to cause them tothermally adhere, or alternatively they may be affixed by an adhesivesuch as FEP.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a typical diametrically adjustable stent.

FIG. 2 is an enlarged schematic representation of the microstructure ofporous expanded PTFE film having a microstructure withuniaxially-oriented fibrils as used to construct Examples 1 and 3.

FIGS. 3A and 3B describe enlarged schematic representations of themicrostructure of porous expanded PTFE film having microstructures ofmultiaxially-oriented fibrils as used to construct Example 2.

FIG. 4 is a transverse cross section of the stent of Example 1 having aluminal layer of porous expanded PTFE film with longitudinally-orientedfibrils and an exterior layer of porous expanded PTFE film withcircumferentially-oriented fibrils.

FIG. 5 is a transverse cross section of the stent of Example 2 having aluminal layer of porous expanded PTFE film with biaxially-orientedfibrils.

FIG. 6 is a transverse cross section of the stent of Example 3 having anexterior layer of porous expanded PTFE film withcircumferentially-oriented fibrils.

FIG. 7 describes a method of collapsing a previously outwardly adjustedballoon-expandable stent.

FIG. 8 describes the fitting of a single tubular sleeve to both theexterior and luminal surfaces of a stent.

FIG. 9 describes the removal a covered, braided wire stent of theself-expanding type from a manufacturing mandrel by everting the braidedwire, thereby placing the covering on the luminal surface of the stent.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a side view of a typical diametrically adjustable stent. Thestent is shown as it would appear implanted into a body conduit with itsdiameter adjusted beyond the collapsed pre-implantation diameter. Whilethe stent shown is made from metal wire, a perforated sleeve havingperforations of suitable shape, size and quantity may also be used.Various suitable stents are described by U.S. Pat. No. 4,776,337 toPalmaz and PCT US 92/03481 to Hess. These stents may be made fromimplantable metals such as titanium, stainless steel, or Nitinol.

The stent may be provided with an exterior covering of porous expandedPTFE film, or a luminal covering of porous expanded PTFE film, or withboth exterior and luminal coverings. Uniaxially-oriented films having amicrostructure of uniaxially-oriented fibrils wherein substantially allof the fibrils are oriented parallel to each other may be used.Multiaxially-oriented films having a microstructure of biaxially ormultiaxially-oriented fibrils wherein the fibrils are oriented in atleast two directions which are substantially perpendicular to each othermay also be used.

FIG. 2 describes an enlarged schematic representation of themicrostructure of nodes 11 connected by fibrils 12 of a film 20 whereinthe uniaxially-oriented fibrils 12 are substantially parallel to eachother. FIGS. 3A and 3B describe enlarged schematic representations ofalternative microstructures of porous expanded PTFE films that may alsobe used for making the present invention. These microstructures havenodes interconnected by fibrils wherein the fibrils are oriented in atleast two directions which are substantially perpendicular to eachother. FIG. 3A describes a microstructure 30 of nodes 11 and fibrils 13and 15 wherein the fibrils 13 and 15 are biaxially-oriented fibrilswhich are oriented in two different directions that are substantiallyperpendicular to each other. Those microstructures may contain somefibrils 17 which are not oriented in the two different directions. FIG.3B describes another alternative microstructure 35 wherein the fibrils21 are multiaxially-oriented fibrils oriented in virtually alldirections within the plane of the sheet material. Either of these filmsmay be made by having been expanded two directions that areperpendicular to each other. The microstructure 30 ofmultiaxially-oriented fibrils 21 may also have been made by beingexpanded in more than two directions as shown by FIG. 3B. Themanufacture of these films is taught by U.S. Pat. Nos. 3,953,566;4,198,390 and 4,482,516.

The fibril lengths of the porous expanded PTFE films referred to hereinwere estimated mean values obtained by examining scanning electronphotomicrographs of these films. For multiaxially-oriented films, theseestimates included consideration of fibrils oriented in all directions.The mean fibril lengths of the films used to construct the intraluminalgrafts of the present invention are preferred to be within the range ofabout 5 to about 120 microns, although fibril lengths beyond this rangemay also be useful.

Wall thickness measurements of intraluminal graft stent coverings weredetermined by cutting away a portion of the covering that covered anopening through the stent wall. The thickness of the sample portion wasmeasured by placing the sample portion between the pads of a Mitutoyomodel no. 804-10 snap gauge having a part no. 7300 frame, and gentlyeasing the pads into contact with the sample portion until the pads werein full contact with the sample portion under the full force of thespring-driven snap gauge pads. Film density measurements were based onthe bulk volume of a film sample using the snap-gauge thicknessmeasurement.

The following examples of intraluminal stent grafts are intended to beillustrative only and are not intended to limit the scope of theinvention to only the constructions described by these examples.

EXAMPLE 1

A Nitinol wire stent 10 (Nitinol Medical Technologies, Boston, Mass.) ofthe type described by FIG. 1 was provided with both a luminal coveringand an exterior covering of expanded PTFE film. This 3 cm long stent wasformed from 0.25 mm diameter Nitinol wire into a tubular shape ofinterlocking hexagons. The luminal and exterior coverings were both madefrom a uniaxially-oriented film having fibrils oriented substantially ina single direction wherein the fibrils were all substantially parallelto each other. The luminal covering was provided with the fibrilsoriented parallel to the longitudinal axis of the tubular stent; theexterior covering was provided with the fibrils oriented substantiallycircumferential to the tubular stent. The film used for both the luminaland exterior coverings was a porous expanded PTFE film having adiscontinuous, porous coating of FEP applied to one side of the porousexpanded PTFE film. Examination of the FEP coated side of the film byscanning electron microscopy revealed FEP on only small portions of thenodes and fibrils at the surface of the film. It was estimated that lessthan 10% of the available node and fibril surface area exposed at thesurface of the film was covered by FEP. The presence of the FEP adhesivethus had little or no adverse effect on the porosity of the porous PTFEfilm.

The FEP-coated porous expanded PTFE film was made by a process whichcomprises the steps of:

-   -   a) contacting a porous PTFE film with another layer which is        preferably a film of FEP or alternatively of another        thermoplastic polymer;    -   b) heating the composition obtained in step a) to a temperature        above the melting point of the thermoplastic polymer;    -   c) stretching the heated composition of step b) while        maintaining the temperature above the melting point of the        thermoplastic polymer; and    -   d) cooling the product of step c).

In addition to FEP, other thermoplastic polymers including thermoplasticfluoropolymers may also be used to make this coated film. The adhesivecoating on the porous expanded PTFE film may be either continuous(non-porous) or discontinuous (porous) depending primarily on the amountand rate of stretching, the temperature during stretching, and thethickness of the adhesive prior to stretching.

The discontinuously FEP-coated porous expanded PTFE film used toconstruct this example was of about 0.01 mm thickness and had a densityof about 0.3 g/cc. The microstructure of the porous expanded PTFEcontained fibrils of about 50 micron mean fibril length.

A 3.0 cm length of film 20 having uniaxially-oriented fibrils waswrapped as a single layer 41 around a hollow, tubular, 1.5 cm outsidediameter mandrel 43 of non-porous PTFE to form a seam 45 as described bythe cross section of FIG. 4. The seam edges 45 overlapped as shown byabout 3 mm. The fibrils of the film were oriented parallel to thelongitudinal axis of the mandrel; the FEP-coated side of the film facedaway from the surface of the mandrel. The Nitinol stent was carefullyfitted over the film-wrapped portion of the mandrel. The 3 cm length ofthe stent was centered over the 3.0 cm length of film-wrapped mandrel.The stent was then provided with an exterior covering 47 of a 3.0 cmwide tape of the film described above by wrapping the tapecircumferentially around the exterior surface of the mandrel so that theedges of the circumferentially-wrapped tape overlapped by about 3 mm toform seam 49. The circumferentially wrapped covering was oriented sothat the FEP-coated side of the tape faced inward in contact with theexterior surface of the stent and the outward facing FEP-coated surfaceof the luminal layer of film exposed through the openings in the stent.Except for the overlapped seam edges 49, the circumferentially-wrappedcovering was only one film layer thick. The uniaxially-oriented fibrilsof the microstructure of the circumferentially-wrapped tape werecircumferentially-oriented about the exterior stent surface.

The film-wrapped mandrel assembly was placed into an oven set at 360° C.for a period of 4 minutes after which the film-wrapped mandrel wasremoved from the oven and allowed to cool. Following cooling toapproximately ambient temperature, the mandrel was removed from thefilm-wrapped stent. The amount of heat applied was adequate to melt theFEP-coating on the porous expanded PTFE film and thereby cause adjacentlayers of film to adhere to each other. Thus the luminal layer of filmwas adhered to the exterior circumferentially wrapped layer through theopenings between the adjacent wires of the stent. The combined thicknessof the luminal and exterior coverings was about 0.025 mm.

The film-covered stent was then chilled in a bath of ice water whilebeing rolled between human fingers applying compression diametricallyacross the stent. This reduced the outside diameter of the stent toabout 0.3 cm. The collapsed stent was then heated by immersion in about40° C. water, thereby increasing the stent diameter to about 1.5 cm. Thefilm covering showed no visible adverse effects from the process ofshrinking and increasing the stent diameter.

EXAMPLE 2

A Nitinol wire stent of the same type used for Example 1 was providedwith a luminal covering of a porous expanded PTFE film having amicrostructure of biaxially-oriented fibrils as shown by FIG. 3A. Thiswas accomplished by wrapping a hollow tubular mandrel of non-porous PTFEwith a layer of porous expanded PTFE film having a continuous(nonporous) coating of FEP with the FEP-coated side of the film facingoutwardly away from the mandrel surface. This film was about 0.02 mmthick; the porous expanded PTFE had a microstructure ofuniaxially-oriented fibrils with the fibrils oriented circumferentiallyabout the exterior surface of the mandrel. The Nitinol stent wascarefully fitted over the film-wrapped portion of the mandrel. Themandrel assembly was then placed into an oven set at 360° C. for fourminutes. After removal from the oven and subsequent cooling, the mandrelwas removed from the stent leaving the wrapped film adhered to theluminal surface of the stent. This film was then peeled from the luminalstent surface, leaving the FEP-coating and some small shreds of residualporous expanded PTFE adhered to the luminal surface of the stent wires.By removing the film and leaving the FEP adhesive on the luminal stentsurface, the film served only as a release substrate for the applicationof the adhesive to the stent surface.

As shown by FIG. 5, the mandrel 43 was then provided with a single layer51 wrapping of a porous expanded PTFE film 35 having a microstructure ofbiaxially-oriented fibrils. This film was of about 30 micron fibrillength, about 0.08 mm thickness, about 0.3 g/cc density and did not havean FEP coating. The biaxially-oriented fibrils were oriented to besubstantially parallel to the longitudinal axis of the mandrel and tothe circumference of the mandrel.

The film was overlapped adequately to form a 2 mm wide, longitudinallyoriented seamline 45 parallel to the longitudinal axis of the mandrel. Asheet of polyamide film was temporarily placed over the surface of theseam and then contacted with the surface of a hand-held iron set at 400°C. to cause the PTFE film seam edges to adhere to each other. Excessmaterial beyond the 2 mm wide seam was trimmed away and discarded. Thestent was again carefully fitted over the film-covered mandrel. Theresulting assembly was placed into an oven set at 380° C. for threeminutes and then removed and allowed to cool, after which the mandrelwas removed from the stent. The porous expanded PTFE film appeared to bewell adhered to the luminal surface of the wire stent by the FEP coatingleft from the first, previously removed, layer of film. The wallthickness of the PTFE film covering was about 0.08 mm.

The film-covered stent was then chilled in a bath of ice water whilebeing rolled between human fingers applying compression diametricallyacross the stent. This reduced the outside diameter of the stent toabout 0.3 cm. The collapsed stent was then heated by immersion in about40° C. water, thereby increasing the stent diameter to about 1.5 cm. Thefilm covering showed no visible adverse effects from the process ofshrinking and increasing the stent diameter.

EXAMPLE 3

A Palmaz stent of the balloon-expandable type (part no. PS30, Johnson &Johnson Interventional Systems, Inc., Warren, N.J.) was adjusted fromits collapsed outside diameter of 3.4 mm to an enlarged outside diameterof 8.0 mm by inserting a tapered stainless steel mandrel followed by astraight 8.0 mm diameter stainless steel mandrel. This stent was thenprovided with a single layer exterior wrapping of the samediscontinuously FEP-coated, porous expanded PTFE coating used for theexterior wrapping of the stent of Example 1. This was accomplished bywrapping the film about the exterior surface of the mandrel with theuniaxially-oriented fibrils of the film microstructure oriented parallelto the longitudinal axis of the stent. This exterior covering 61 isdescribed by the transverse cross section of FIG. 6. A 2 mm wide seam 45was formed from the overlapped edges of the porous expanded PTFE film 20by temporarily placing a thin sheet of polyamide film over these edgesand applying heat from a hand-held iron with a surface temperature ofabout 400° C. The film-wrapped stent 65 was then placed into an oven setat 380° C. for 3 minutes, after which it was removed and allowed tocool. The film appeared to be well adhered to the exterior surface ofthe stent. The wall thickness of the film covering was about 0.01 mm.The enlarged stent was then collapsed by the following process.

A series of 20 cm long 6-0 sutures were tied individually to each of theclosed metal stent openings adjacent to one end of a stent. Thefilm-covered stent was provided with a temporary non-adhered additionalwrapping of longitudinally-oriented film without FEP and having amicrostructure of uniaxially-oriented fibrils. This temporary wrappingwas intended as a dry lubricant. As described by FIG. 7 which omits theexterior film covering for clarity, the enlarged stent 71 was thenpulled by these sutures 77 through a tapered die 75 of round crosssection and 2.5 cm length, the die having a tapered orifice with a 9.5mm diameter bore at its entrance 78 and a 4.5 mm diameter bore at itsexit 79. The result was that the stent was collapsed back to an outsidediameter of 4.5 mm. The lubricity of the temporary covering of porousexpanded PTFE film aided in making it possible to pull the stent throughthe die. This temporary covering was removed after completion of thecollapsing process. It is anticipated that the use of a tapered diehaving an appropriately sized, smaller diameter exit bore would resultin collapsing the stent to its original collapsed diameter. Thefilm-covered stent was again enlarged to a diameter of 8 mm using aballoon catheter followed by a tapered stainless steel mandrel. Thecovering of porous expanded PTFE film appeared to be fully intact afterthe collapsing and enlarging of the film-covered stent.

Stent coverings may be affixed to a stent surface by variations on thismethod. For example, a tubular sleeve may be made from a film of porousexpanded PTFE and inverted back into itself and fitted over the innerand outer surfaces of a stent as shown by FIG. 8. The inner 83 and outer85 portions of the tubular sleeve 81 may be thermally adhered to eachother through the openings in the stent wall, or may be adhered to thestent surfaces by an adhesive such as FEP, or may be affixed to thestent by suturing the open ends 87 of the tube together.

EXAMPLE 4

A long length of 0.07 mm diameter single strand 304 stainless steel wirewas provided with a single layer, approximate 1 mm overlap covering ofporous expanded PTFE film by helically wrapping the wire with a narrowtape cut from a sheet of porous expanded PTFE film. The tape used was 6mm wide, 0.01 mm thick, 0.3 g/cc density, and had uniaxially-orientedfibrils of about 50 micron fibril length. This tape-covered wire wasthen heated by pulling the wire through the 0.14 mm diameter orifice ofa 2.5 cm long die heated to 400° C., at a rate of 1.5 meters per minute,thereby adhering the overlapped edges of the tape together and therebyadhering the tape to the wire. This wire was then cut into shorterlengths and spooled onto 16 bobbins. These bobbins were used to supplythe wire to a model D-5600 Steeger braider.

A 12 meter length of 1.75 mm diameter non-porous PTFE mandrel was thenfed into the braider where a braided covering of the above wire wasapplied at a density of about 16 picks/cm. An additional covering oftape cut from a sheet of porous expanded PTFE film was then helicallywrapped over the surface of the wire-braided PTFE mandrel. The tape usedfor this helical wrapping was of 0.01 mm thickness, 0.3 g/cc density,about 50 micron fibril length and 12 mm width. Adjacent edges of thehelical wrapping were overlapped by approximately 1 mm. The wire-braidedTeflon mandrel was then placed into an oven set at 380° C. for fourminutes, after which it was removed and allowed to cool. As shown byFIG. 9, the wire-braided stent 91 with the exterior covering of porousexpanded PTFE tape was then removed from the non-porous PTFE mandrel 93by folding the ends 95 of the braided wires back on themselves andpulling on these everted ends. The exterior covering of porous expandedPTFE film is omitted from FIG. 9 for clarity. By applying tension onthese everted ends in a direction parallel to the longitudinal axis ofthe mandrel and from the everted end back toward the opposite,non-everted end, the entire braided construction was everted andsimultaneously removed from the mandrel. This everting process ofremoving the braided assembly from the mandrel resulted in the helicalwrapping of film being located on the lumen of the stent. Thisconstruction offered good self-expanding characteristics in that whenlongitudinal tension was placed on the stent, the length of the stentincreased and the diameter decreased. Upon release of tension, the stentimmediately recovered its previous shorter length and larger diameter.This film-covered stent is therefore expected to be useful as aself-expanding stent.

1. A method of making a tubular intraluminal graft comprising: a)providing a length of wire; b) providing the length of wire with a firstpolymeric covering to create a covered wire; c) braiding the coveredwire to create a tubular form having a wall with a multiplicity ofopenings therethrough; and d) providing the tubular form with a secondpolymeric covering which covers at least some of the openings.
 2. Amethod according to claim 1 wherein said first covering is porouspolytetrafluoroethylene.
 3. A method according to claim 2 wherein saidsecond covering is porous polytetrafluoroethylene.
 4. A method accordingto claim 1 wherein said second covering is porouspolytetrafluoroethylene.
 5. A method according to claim 1 wherein saidfirst covering is expanded polytetrafluoroethylene.
 6. A methodaccording to claim 5 wherein said second covering is expandedpotytetrafluoroethylene.
 7. A method according to claim 1 wherein saidsecond covering is expanded polytetrafluoroethylene.